1. Field Of The Invention
This invention pertains to devices using magnetic fields for measuring the position and orientation of receiving antennae with respect to transmitting antennae.
2. Description Of The Prior Art
The concept of using transmitting and receiving antennae with electromagnetic coupling for measuring position and/or orientation is well know for military target sighting applications. In this application a gunner is seated in the cockpit of an aircraft and the receiving antennae are located on his helmet. Transmitter antennae are located a few inches away usually on the plastic canopy directly behind his head. As the gunner sights a target through a sighting crosshair affixed to his helmet, the receiving antennae pick up signals generated by the transmitting antennae. These signals are processed by a computer to determine the location and orientation of the helmet and then to point the armament in the same direction as the helmet mounted sight.
Prior art magnetic field position and orientation measuring systems are typified by the following referenced patents U.S. Pat. Nos: 3,868,565 (Kuipers), 4,054881 (Raab), 4,287,809 (Eglie et al), 4,849,692 and 4,945,305 (Blood). These systems utilize dipole transmitting antennae which generate either AC or DC magnetic fields that are measured by receiving antennae located on the user. The received signals are converted into position and orientation using mathematics that describes the transmitted dipole fields. These systems have the following characteristics in common:
1. The equations used to determine position and orientation are based on the signals being generated by dipole transmitting antennae.
2. The transmitter consists of two or three individual dipole antennae that are collocated about a common center and whose axes are ideally orthogonal to each other.
3. The characteristics of the transmitted field are such that the signal strength falls off rapidly by one over the cube of the distance to the receiver.
4. The receiver operates outside the transmitter coils at some distance, it does not operate within or between the transmitter coils.
5. If there are electrically conductive metals (such as aluminum) near the transmitter then the time varying components of the transmitted fields will induce eddy currents in these metals which will in turn generate a magnetic field which will distort the transmitted field. As the receiver to transmitter separation increases the resultant errors in the computed position and orientation of the receiver increases. This eddy current generation is an acute problem for systems that utilize AC transmitted signals while systems that use a pulsed DC transmitted signal are not as sensitive.
6. If there are magnetically permeable metals (such as carbon steel) near the transmitter then the magnetic flux generated by the transmitter will be distorted by this permeable material. For a given steel structure surrounding the transmitter and receiver, the resulting errors in position and orientation will increase as the distance between the transmitter and receiver increases. The magnitude of this permeable material error is similar for both AC and DC systems.
For a transmitting loop antenna to generate true dipole fields at a given location, the antenna must be either infinitesimally small or infinitely far from the field measurement location. If true dipole field conditions do not prevail then the computed position and orientation will be in error. To make the fields more dipole like, Voisin in U.S. Pat. No. 5,172,056 utilizes a unique transmitter and receiver antenna coil winding geometry while Jones in U.S. Pat. No. 4,737,794 discloses a mathematical method to make the field from a circular loop transmitter look like a dipole when the receiver is at distances greater than the radius of the transmitter coils.
Large area antenna have been disclosed in a paper titled "Headsight Television System Provides Remote Surveillance" by Charles P. Comeau and James S. Bryan in Electronics, Nov. 10, 1961. This system requires six antenna to generate a uniform field over the central part of a room where it is desired to measure head orientation.
In a newer application of the technology in the field of computer animation it is desired to animate computer generated images of cartoon characters, robots, or animals. To accomplish this, one or more actors with several receivers attached to their arms, legs, and torso perform the desired sequence of movements in a room equipped with the transmitter. The receiver's position and orientation measurements are captured by the animation computer and used to make the computer generated image follow the actors's motions.
FIG. 1 shows the use of prior art trackers in this application. Typically, one or more actors 1 are located in a room with walls, floor, and ceiling 2. The room contains two or three transmitting dipole antenna 3 centered about a common origin. The antenna is mounted on some type of support structure 4 in the area where the actor will be moving. On the actor are one or more receiving antenna 5 used to measure his body motion. The transmitter and receiver antennae are connected to an electronics box 6 via wire cables 7 where the signals are transformed into receiver position and orientation for use by the computer animation system.
These prior art systems suffer from the following problems:
1. The strength of the transmitted dipole field drops off as 1/R.sup.3, where R represents the distance from the transmitter to the receiver. Because of this rapid drop off in signal, the distance that the actor can move about the transmitter antennae is limited. To overcome this limitation several transmitters must be used to provide the desired coverage or a lot of power must be used to overcome the 1/R.sup.3 drop in signal.
2. As the actor moves farther from the transmitter, the position and orientation measurements exhibit increased errors due to the conductive and permeable metals used in the construction of the building thereby limiting the area over which the actors can move.
An object of this invention is to provide for the first time an electromagnetic position and orientation measuring system that can provide a large operational volume without the need for multiple transmitters or a high power transmitter.
It is also an object of this invention to provide a measurement system that can provide a large operational volume inside a metal building with smaller measurement errors than prior art systems resulting from the interaction of the metal with the signal.
It is further an object of this invention to provide a transmitting antenna configuration and signal processing method that can be adapted to many different antenna mounting environments by eliminating the prior art requirements of concentric, near-orthogonal, and dipole like transmitting antenna.